Hi, Geri. Usually, unless the maker specifies otherwise, A=440 is the way to go for most harps. For period instruments it’s a lot less, sometimes A=415.
That probably does explain a lot of your breakage problems.
You probably already know this, but just in case…it will probably take the wire harp longer to settle into its new tuning than the nylon strung harp (and even that can take a while.
A=440 is generally the most common. I also think it might depend on the harp slightly as well. I have one harp that sounds best when tuned to 440, but another that sounds “thunky” and hollow at 440. It really sings and resonates when tuned to 442 though. I know, I know, it’s a minimal difference in pitch, but I can hear it, and it really makes a tonal difference on my Prelude. So you might want to try 440, then play around with it a little bit.
Geri has your string breakage lessened? I was concerned as 13 htz high is not excessive and your string breakage should not be occurring. When I am settling in a new string on a test jig I frequently bring it a full step and one half over and allow it to settle back down over night without breakage.